The present invention relates to techniques for transmitting data blocks of variable size on synchronous links, these data blocks serving to form modulated-signal blocks which are then sent on multiplexed physical channels.
It has applications in particular in systems for radio communication with mobiles which can operate in packet mode, such as GPRS (“General Packet Radio Service”) networks.
On multiplexed physical channels, the modulated-signal blocks are often sent according to a frame structure defining successive periods for sending blocks on each channel. For example, the periodicity of the GPRS radio blocks is 20 ms.
The modulated-signal blocks can form the subject of variable protection against transmission errors. This can be achieved by adjusting the rate of the channel coder generally provided in the transmission chain, either by selecting the code applied from among several block codes or convolutional codes, or by setting a degree of puncturing of the code. Another possibility is to vary the level of protection by choosing a more or less robust modulation: for a given symbol duration within the modulated signal, a binary modulation, for example, will provide greater protection than a quaternary or 8-ary modulation.
Owing to the variable protection against transmission errors, the modulated-signal blocks are formed from data blocks of variable size. In certain systems, the construction of these data blocks of variable size and their transformation into modulated-signal blocks by coding and modulation are not performed at the same spot, so that these blocks of variable size must be transmitted from their source to the coder.
In the example of downlinks in GPRS networks, the data source which constructs the variable-size blocks is called the PCU (“Packet Control Unit”), and the coder and the modulator are situated in the transceiver base stations or BTS. In the frequent case where the PCU is remote from the BTS, the uncoded blocks are transmitted on a time-division multiplexing synchronous link. These links rely on a standardized technique of pulse code modulation (PCM). Each physical channel on the radio interface is associated with a dedicated channel on the synchronous link, this channel consisting of a 16-kbit/s PCM subpath. A PCM path, called DS0, ensures a throughput of 64 kbit/s, at the rate of one byte every 125 μs. The PCM circuits support up to 32 DS0 paths in the case of a type E1 synchronous interface (2.048 Mbit/s) or up to 24 DS0 paths in the case of a type T1 synchronous interface (1.544 Mbit/s).
A problem posed by systems of this type is that the capacity of a dedicated channel of the synchronous link over a block sending period may be insufficient with regard to the blocks having the largest sizes, that is to say those which contain the least-protected data. A fixed allocation of several channels of the synchronous link to each physical channel is possible, but it leads to an expensive overrating of the transmission capacity.
An aim of the present invention is to propose a mode of transmission of data blocks of variable size which avoids the above problem while limiting the complexity and the cost of the equipment.